The present invention relates generally to rotating seals, and, more specifically, to multistage brush seals.
Various forms of engines or motors include various forms of seals therein specifically configured for separating relatively high and low pressure regions thereof. For example, gas and steam turbine engines have various stages therein in which air, combustion gases, and steam travel downstream with changes of pressure and temperature thereof.
Turbine engines are made as large as practical for maximizing output work and performance efficiency. Large turbines also have correspondingly large pressure drops of the fluid flow therethrough which requires suitable sealing during operation.
In a labyrinth seal, a row of annular seal teeth is disposed closely adjacent to a cooperating land for relative rotation therewith, with a radial clearance therebetween for reducing the likelihood of undesirable rubbing therebetween. Another form of seal typically found in turbine engines is the brush seal in which a pack of bristles is mounted between supporting plates, with the distal ends of the bristles extending freely therefrom for effecting a rotary seal with the adjacent land. The bristles are inclined from the land and may form a small clearance therewith or may be in interference contact therewith.
Brush seals offer the ability to effectively seal the very high pressures in a turbine engine, for example, while maintaining stability during relative rotation with the land and having a suitably long life during operation.
Experience has shown that a single stage brush seal has a practical limit of sealing differential pressure up to about 400 psid, with a suitable safety factor for correspondingly reducing that limit. Over-pressurization of the brush seal will cause excessive leakage, plastic deformation thereof or fatigue failure in short time.
Brush seals may be disposed in series, but again experience has shown that their ability to seal very high differential pressures in excess of 400 psid is again limited to substantially less than twice the 400 psid limit, correspondingly reduced by the safety factor, for each seal in view of the practical variation in load sharing therebetween. And, experience has additionally shown that additional series brush seals in excess of two have no practical capability of sealing increased differential pressure imposed across those multiple stage brush seals.
In two or more stages of identical brush seals, the first stage is capable of sharing a minority of the total pressure load across the brush seal assembly, with the last brush seal carrying a majority of the total pressure loading. Accordingly, the sealing capability of the multistage brush seals is limited by that last stage, and correspondingly limits the maximum differential pressure which the seals may safely carry during operation.
It is known to vary the seal clearances with the adjacent land to vary the load sharing capability of each seal stage. However, the specific value of the seal clearance affects the total sealing capability, with larger clearances decreasing loading capability, while smaller clearances increase loading capability.
However, small clearances subject the bristles to increased friction rubbing with the land during transient operation of the engine, with friction rubbing correspondingly heating the components. In a typical example, the land is defined by the outer perimeter of a rotary shaft, and the brush seals are stationary. Friction rubbing of the bristles with the shaft causes local heating thereof and corresponding thermal expansion which can adversely affect stability of the rotating shaft. As the shaft thermally expands under rubbing, the rubbing friction forces further increase for further increasing friction heating of a shaft. And, the shaft is subject to undesirable instability, such as wobbling, which can require emergency shut down of the entire engine.
The geometric configuration of the bristles also affects the maximum sealing capability thereof. Soft or flexible bristles are subject to increased bending under differential pressure, and hard or stiff bristles increase the friction heating under rubbing with the land. Furthermore, the differential pressure exerted across the pack of bristles effects blowdown therein in which the inclined bristles deflect slightly radially inwardly, which correspondingly increases friction heating during rubbing with the land.
In view of these interrelated operation effects of brush seals, conventionally known multistage brush seals are limited to two brush seals in series, with a current practical limit of 550 psid total differential pressure based on a suitable factor of safety therein. And, the ability to achieve load sharing between the multiple seals stages is limited by bristle rubbing and rotor stability.
Accordingly, it is desired to provide an improved multistage brush seal for increasing the maximum load carrying capability thereof.